Sand moulding mixtures



United States Patent U.S. Cl. 10638.35 5 Claims ABSTRACT OF THE DISCLOSURE A sand moulding mixture including sand and water and an additive including natural clay impregnated with a mineral oil.

This invention relates to moulding mixtures useful in the preparation of green sand moulds which are used for casting metal. The term sand used herein is intended to embrace all sands knewn per se for the production of foundry moulds and cores. It thus includes not only silica sand but such alternatives as zircon or olivine sands.

A green sand mould is a mould which has not been subjected to a baking or hardening process before it receives molten metal in the casting operation. Such moulds are prepared from moulding mixtures which comprises essentially sand, a binding additive and water. This mixture when compounded in the correct proportion is of sufiicient strength to enable a mould to be prepared which can then be used in the casting process. Optionally other additives may be added to green sand moulding mixtures in order to improve the moulding properties of the sand or to modify the behaviour of the mould during the casting operation. Two additives which have been used in order to modify the behaviour of the green sand mould during the casting operation, especially when iron is being cast, are coal dust and pitch. Coal and pitch will liquify at least partially, when subjected to the heat of the molten metal, thus allowing expansion of the sand grains without deformation of the mould. On continued exposure to the molten metal both the coal dust and the pitch tend to decompose to produce reducing gases and a fixed-carbon/ ash residue. This residue may contribute some binding action to aid the coherence of the moulding material which, however, does not become rigid and is capable of accomodating further expansion of the moulding sand grains. The reducing gases produced by the decomposition of the coal or pitch tend to produce a cushion between the Walls of the mould and the molten metal, thus inhibiting both oxidation of the molten metal surface and the production of fayalite (iron silicate) which has a tendency to be formed by reaction between iron, iron oxide and sand.

Adhesion of the metal to the mould walls is reduced and there is a consequent decrease in the incidence of surface defects on the resultant casting.

There have now been discovered green sand moulding mixture additives which appear to function in a similar manner to coal dust or pitch but are more effective and may thus be used in smaller quantities, and are more convenient than coal dust.

According to the present invention there is provided a sand moulding mixture which comprises sand and water and, as an additive thereto, a solid substance impregnated with a liquid which is substantially non-volatile and stable at ordinary temperatures but which, at high temperatures, will generate a non-oxidising atmosphere by volatilisation or decomposition, the said additive being in particulate form and the said mixture including a proportion of clay.

3,440,066 Patented Apr. 22, 1969 It is to be understood that the solid substance of the additive may itself be clay but that if the additives does not include clay then some clay should be additionally included in the mixture by reason of its presence as a natural constituent of the sand used or by reason of its specific addition.

According to a preferred form of the invention, at least part of the solid substance of the additive is clay, and while any type of clay can be employed it is generally preferred for this purpose to use a purer form of clay such as bentonite, kaolin or fullers earth. However, it is to be understood that providing some clay is present in the final sand mixture it is possible to employ other substances than clay for the absorbtion of the liquid in the additive, e.g. kieselguhr, wood flour or porous refractory material such as grog.

A preferred additive for use in the present invention is an oil-impregnated clay, e.g., a clay containing at least 5% and preferably 10 to 20% by weight of an oil, for instance a mineral oil such as crude or waste lubricating oil. Such an additive may be employed in a proportion of 0.5 to 20%, preferably 1 to 5%, by weight of the final sand moulding mixture.

The successful use of oil-impregnated clay is unexpected because although it is known to add fuel oil to moulding sand to prevent it sticking to the pattern, such a practice has been avoided because generally it has been found that the heat of the molten metal entering the mould causes a very rapid evaporation of the oil and, consequently, a great agitation of the molten metal or even ejection of molten metal from the mould. When oilimpregnated clay is used it is believed that rapid evaporation and decomposition of the oil is prevented by reason of the oil being absorbed in the clay. It is thought that such oil-impregnated clay functions as a green sand additive in much the same wa as coal dust or pitch, softening at 400 C. to 700 C. and decomposing with continued exposure to the molten metal to produce a cushion of reducing gases and a fixed carbon residue. However oil-impregnated clay is more effective as a moulding sand additive; for example 2% by weight of oil-impregnated clay is as effective as 5% by weight of coal dust when preparing large castings. Furthermore, if the sand employed is one to which clay must be added in order to secure adequate moulding properties it is possible in the present invention to reduce the amout of such added clay by an amount up to and including an amount commensurate with the quantity of oil-impregnated clay which is added. Moreover, it is a noticeable phenomenon, when using coal dust additions to recycled foundry sand, that the carbon content of the sand tends to build up in the form of coke. Such an increase in coke content makes it necessary to increase the water addition to the moulding mixture. This has a deleterious affect on the moulding properties of the sand and ultimately leads to the incidence of surface defects in the castings produced. The use of oil-impregnated clay additive does not suffer from this same disadvantage and water additions may be kept at a constant level, although recycled sand is employed.

A further advantage of using oil-impregnated clay as a green sand moulding mixture additive lies in the fact that the presence of the oil appears to improve the moisture retention properties of the mixture so that the plasticity of the clay bond is maintained and the mixture remains in a workable condition for a longer period.

In foundry practice the use of coal dust leads to difiiculties in handling and substantial losses of the coal dust fines via air conditioning systems. Such losses have been estimated to be as high as 15% by weight. The use of oil-impregnated clay of course avoids such losses and the material is much more easily handled.

Various other substances may be included in the addi tive for special purposes. Thus it may sometimes be desirable to include further mineral oil, especially if the oil content of the oil-impregnated clay is rather low. Alternatively, or in addition there may be added a quantity of pelleted pitch, an ingredient which tends to confer dimensional stability on the moulding mixture. Substances for improving the bond characteristics of the mixture, e.g., bentonite, also may be included.

In order to facilitate the admixture of water with the moulding mixture containing the oil-impregnated clay, a wetting or emulsifying agent of conventional character may be employed, e.g., an anionic surface active compound such as a long-chain alkyl sulphate or an alkyl-aryl sulphonate or naphthalene sulphonate. Numerous emulsifying agents are known and are commercially available for a wide variety of purposes, e.g., in the textile art, and are of general utility in the compositions of the present invention.

In the use of recycled sand there is a tendency for the clay constituent of the sand to become inactivated by continued use and this tendency may be overcome by increasing the gel-forming properties of the clay by means of a suitable addition for that purpose, e.g., magnesium oxide, the effect of which is to increase the green strength and shatter index of the total mixture.

It is also sometimes convenient to include an ingredient which acts to buffer the pH to at least 7.0. Suitable buffering agents are for example tetrasodium pyrophosphate (the presence of which ensures adequate dispersion of the clay in the moulding composition and which appears noticeably to increase the dry strength of the moulding mixture) and sodium carbonate.

Other additives may be used in conjunction with the oil-impregnated clay according to our invention. For example, wood fiour may be mixed with the oil-impregnated clay to improve its absorbency and allow the additive to possess a high volatile (i.e., oil) content and yet to be a free-flowing powder.

The present invention includes not only the sand moulding mixtures described above but also the additives per se which characterise such mixtures, i.e., additives which consist of a solid substance impregnated with a liquid which is substantially non-volatile, and stable at ordinary temperatures but which at high temperatures generates a non-oxidising atmosphere by volatilisation or decomposition. More specially the invention includes such additives which further contain a surface active compound, and more particularly oil-impregnated clay in particulate form and the said product further containing a surfaceactive agent.

The following examples will serve to illustrate the Oil-impregnated clay (40% oil) 3 The above two mixtures were used to prepare green sand moulds to receive molten cast iron for the production of castings therefrom. Despite the omission of a proportion of clay and water from mixture B, the moulding properties of both compositions were similar and the surface of the iron castings produced were equally satisfactory, being free from surface defects.

Although the oil-impregnated clay additive of the above example functioned in a suitable manner, its continuous use as an additive to recycled foundry sand could result in the moulding mixture becoming repellent to water addition. An improved additive for such recycled moulding mixture is described in the following example.

Example 2 The following additive composition was prepared, in percentage by weight:

Percent Oil-impregnated clay (40% oil) Pelleted pitch 12 Emulsifying agent 0.75 Magnesium oxide l Tetrasodium pyrophosphate 1 Sodium carbonate 0.25

Three parts by weight of this additive composition were added as a direct replacement for the three parts by weight of oil-impregnated clay specified in Example 13 above. Once again, moulds were prepared from the mixture and the castings produced therein were shown to be equally sound and free from surface defects as those resulting from the previous example.

Although the invention has been described by direct reference to the production of green sand moulds, it is to be understood that the additive compositions of the invention may be used in the production of dry sand moulds providing that the oil constituent is of a suitable type and will not be evaporated during the drying operation.

I claim as my invention:

1. In a process of forming a foundry mold composition consisting essentially of sand and water as the liquid medium, the improvement comprising the addition of a preformed particulate solid substance comprising natural clay impregnated with at least 5% by weight of mineral lubricating oil, said preformed particulate solid substance comprising 0.5 to 20% by weight of the foundry mold composition.

2. The process of claim 1 wherein the clay is selected from the class consisting of bentonite, kaolin and fullers earth.

3. The process of claim 1 containing a surface active agent. 1

4. The process of claim 1 containing magnesium oxide.

5. The process of claim 1 containing a pH buffering agent bulfering its pH to a value of at least 7.0.

References Cited UNITED STATES PATENTS 1,514,731 .1-1/ 1924 Rosenbaum 106-38.7 XR 2,813,035 11/1957 Sauter et al. 106-382 XR 2,885,360 5/1959 Hayden et al. 10638.24 XR 2,920,970 1/1960 King et al. =10638.8 XR 3,027,265 3/ 1962 Miericke 106-38.7 XR 3,278,316 10/1966 Sawyer l06--38.6

JULIUS FROME, Primary Examiner.

L. HAYES, Assistant Examiner.

U.S. Cl. X.R. l06-38.3, 38.8 

